The present invention generally relates to a semiconductor laser device for emitting blue laser radiation, which is applicable as a laser light source to an optical information storage medium such as an optical disk. More particularly, the present invention relates to a semiconductor laser device that can prevent spontaneous emission from leaking out of the device and also relates to an optical disk apparatus and optical integrated unit using such a device.
A nitride semiconductor laser device, which is made of nitride semiconductors containing Group III elements like aluminum (Al), gallium (Ga) and/or indium (In), emits blue laser radiation in a wavelength range from 400 to 500 nm when its active layer is made of an InGaN compound. Currently, a red-light-emitting semiconductor laser device, which emits laser radiation in the red part of the spectrum with a wavelength of 650 nm, is applied to recording or reproducing information from a DVD. However, if such a blue-light-emitting semiconductor laser device is actually applied to a DVD, then the storage capacity of the DVD can be increased threefold or more. Specifically, the storage capacity of a disk per side with a diameter of 12 cm can be raised to as high as 15 gigabytes or more. The implementation of such blue-light-emitting semiconductor laser devices is in high demand. This is because a high definition (HD) video signal can be reproduced from a disk with a storage capacity of 15 gigabytes for as long as two hours or more according to the MPEG-2 standard, an international standard of moving picture compression. A nitride semiconductor laser device, which can oscillate successfully at such a short wavelength (i.e., in the range from about 400 to about 450 nm), includes an InGaN active layer and AlGaN cladding layers.
The InGaN active layer and AlGaN cladding layers are usually formed by growing InGaN and AlGaN crystals over a substrate. Accordingly, a GaN substrate, which has the same crystal structure as InGaN and AlGaN and a lattice constant almost equal to those of InGaN and AlGaN, is best suited to growing these layers with minimized crystal imperfections. In the current state of the art, however, a GaN substrate with a size of 10 mm or more on each side is not available. Thus, a sapphire substrate (made of single crystalline Al2O3) is generally used as a substitute substrate. Although sapphire has a lattice constant that is different from that of GaN by as much as 14%, sapphire is still appropriate for the growth of nitride semiconductor crystals because sapphire has the same crystal structure as GaN and is stable enough even at an elevated temperature of 1000° C. or more.
In semiconductor laser devices in general, not just in nitride ones, when current exceeding the threshold value thereof is injected into the p- and n-side electrodes thereof, electrons and holes, which have been created from the injected current, are recombined to emit radiation, which is strongly confined in the active layer. As a result, the current is amplified, continuous oscillation occurs and the radiation is emitted as laser radiation through the emissive end facet of the active layer.
When the active layer of a nitride semiconductor laser device is made of InGaN mixed crystals with an In mole fraction of 15%, bluish purple laser radiation is obtained at an oscillation wavelength of 410 nm. In that case, a pair of cladding layers with a refractive index smaller than that of the active layer should be provided over and under the active layer, and the energy gap between the active layer and the cladding layers should be 0.4 eV or more. These cladding layers are usually made of AlGaN mixed crystals with an Al mole fraction of 7%.
Considering its principle of operation, however, the blue laser radiation emitted from the prior art nitride semiconductor laser device is harder to detect than the red laser radiation, and therefore the signal-to-noise ratio (SNR) obtained at a light-receiving device such as a photodetector is usually low. Thus, compared to the red-light-emitting semiconductor laser device, it is much more necessary for the nitride semiconductor laser device of the short-wave-emission type to reduce the noise produced from the device.